Add 'static mut' items to the language
Support foreign 'static mut' variables as well
Fix 'static mut' tests
auto merge of #7291 : alexcrichton/rust/static-mut, r=huonw

This adds both `static mut` items and `static mut` foreign items. This involved changing far less code than I thought it was going to, but the tests seem to pass and the variables seem functional.

I'm more than willing to write more tests, so suggestions are welcome!

Closes #553